The present invention relates to a method of controlling the operation of a wind-turbine and a wind turbine for use in said method of the kind set forth hereinafter.
In wind turbines of this kind, it is known to control the pitch of the blades of the wind turbine in dependence on measured parameters such as wind speed, wind turbulence, i.e. varying wind speed, etc., in order to optimize the operation of the wind turbine with respect to producing as much energy as possible under the varying weather and wind conditions. The conventional procedure for dimensioning the blades of the wind turbines has been to dimension the blade to withstand the mechanical loads which they are subjected to during normal operation, taking necessary safety margins into account. At very high wind speeds, the wind turbines are normally stopped in order to avoid mechanical overload. During such stop periods, the yaw control is still active, keeping the rotor plane perpendicular to the wind direction, but the blades are pitched out of the wind and rotation is stopped. When the wind speed is again reduced below a certain limit, the wind turbine is started again.
From U.S. Pat. No. 4,339,666 it is known to control the pitch of the blades of a wind turbine in such a way, that the safe operation is secured, i.e. controlled to keep maximum loads on rotor, gearbox and generator below the maximum allowable values. This patent describes the use of average wind velocity and turbulence factor as the primary controlling parameters. This has the disadvantage, that variations in wind speed over the surface of the rotor (wind shear) are not taken into account.
From DE 197 31 918 it is further known to control the pitch of the blades of the wind turbine in dependence of measured mechanical loads on the blades by controlling the pitch in such a way that the mechanical loads are maintained below certain limits during operation.
However, none of these documents indicate the possibility of using the measured mechanical loads to deduce information on the positions of the blade tips and using this information to influence the control of the wind turbine in order to maintain a certain safety distance between the blade and tower.
It is the object of the present invention to provide a method of controlling the operation of a wind turbine and a wind turbine for use in said method of the kind referred to above, with which it is possible to perform the controlling of the wind turbine in such a way, that a certain safety distance can be maintained between the blade tips and the tower, and this object is achieved with a method of said kind, which according to the present invention also comprises the features set forth hereinafter, and with a wind turbine of said kind, which according to the present invention also comprises the features set forth hereinafter. With this arrangement, the controlling function in the wind turbine has access to the instantaneous mechanical loads on the blades and can control the wind turbine in such a way that the position of the tips of the blades deduced from the measured mechanical loads is taken into consideration for setting the control parameters, thus making it possible to keep a certain safety distance during operation. Due to the fact that the controller has access to the measured mechanical loads on the blades, the controller can control the wind turbine close to the mechanical load limits of the blades without any risk of exceeding these limits, whereby the control can be optimized with respect to maximizing the energy output to a higher degree than in a situation in which the optimization is limited by the possible, unknown statistical variation of the wind field during operation, which may be highly dependent on weather and wind conditions, and varying from top to bottom position of the blades (wind shear). This control of the wind turbine close to the mechanical load limits is in the following description designated aggressive control.